Out of the antenna units used in the radio capable of handling multiple bands in the prior art, there is the antenna unit that a feeding point is constructed in a mode of multiple coaxial lines and another coaxial line is connected to the outer conductor, and then the transmitter/receiver is connected to the transmitting/receiving antennas via each center conductor respectively (see Patent Literature 1, for example).
A configurative view of a multi-band antenna in the prior art is shown in FIG. 21. A reference numeral 1 is a ground conductor plate 1, 2 is a first radio circuit, 3 is a second radio circuit 3, 21 is an upper antenna element, 22 is a center conductor, 23 is a lower antenna element, 24, 25, 26 are outer conductors, and 27 is a center conductor of another coaxial line.
For example, a double coaxial line is provided to pass through the ground conductor plate 1 such as an outer wall surface of a high-speed moving body, or the like, and the upper antenna element 21 of the center conductor 22 in a plurality of transmitting/receiving antennas is connected to a center point of the antenna composed of a conductive circular plate, for example, and also the outer conductor 24 is connected coaxially to the lower antenna element 23, e.g., a center portion of the antenna also composed of the conductive circular plate. In such connecting state, the center conductor 22 is connected to the first radio circuit 2 and the center conductor 27 of another coaxial line connected to the outer conductor 24 is connected to the second radio circuit 3. Thus, the good antenna characteristic can be achieved at a plurality of frequencies.
Patent Literature 1: JP-A-6-350332